In general, safety standards require a variety of transformers to employ a structure in which a predetermined insulation distance (creepage distance) is ensured between a primary coil and a secondary coil.
On the other hand, an inverter transformer that causes a cold-cathode tube incorporated as a light source in a liquid crystal display to discharge and emit light boosts the voltage inputted to a primary coil to a high voltage ranging from 1000 to 2000 V in a secondary coil and outputs the high voltage to the cold-cathode tube.
An inverter transformer is therefore required to ensure insulation between the primary and secondary coils by providing a longer insulation distance therebetween than that provided in a typical low-voltage transformer. In a high-voltage isolation transformer in which a high voltage of several hundreds of volts is inputted to the primary coil, in particular, the insulation distance required between the primary and secondary coils is further longer.
In addition to the above, even if insulation is ensured between the primary and secondary coils in a high-voltage producing transformer of this type, a core that is disposed around the coils to form a closed magnetic path is not an insulator and, therefore, desired insulation performance is not obtained in the presence of the core.
On the other hand, for example, Patent Document 1 proposes a transformer including a bobbin having a through hole drilled through a central portion thereof and having primary and secondary coils wound around the outer circumference thereof and a pair of core members, parts of which are inserted into the through hole in the bobbin and abut each other inside and outside the through hole. In the transformer, an insulating member is provided between at least one of the pair of core members and the through hole of the bobbin.
According to the thus configured transformer, since the insulating member is provided between at least one of the pair of core members and the through hole of the bobbin, the creepage distance between the pair of core members and the primary and secondary coils can advantageously be extended.
In the transformer described in Patent Document 1, however, it is necessary to provide the insulating member not only between at least one of the core members and the through hole in the bobbin but also across the total length of the core member in order to achieve a desired advantageous effect, resulting in a problem of a complicated shape and structure of the insulating member, for example, when the core member is an E-shaped core.
Further, since the insulating member is provided between the core member and the through hole, the through hole in the bobbin needs to be larger than a conventional size, resulting in an increased size of the overall transformer, which is disadvantageously against a recent demand for size reduction.
As a result, there has been a strong demand in recent years to develop a high-voltage transformer whose overall size is not increased even when a high voltage is inputted to the primary coil and which can ensure insulation performance required between the primary and secondary sides.    Patent Document 1: Japanese Patent Laid-Open No. 2002-141229